ALBERT

Notizen: The purpose of deconvolution is to retrieve the reflectivity from seismic data. To do this requires an estimate of the seismic wavelet, which in some techniques is estimated simultaneously with the reflectivity, and in others is assumed known. The most popular deconvolution technique is inverse filtering. It has the property that the deconvolved reflectivity is band-limited. Band-limitation implies that reflectors are not sharply resolved, which can lead to serious interpretation problems in detailed delineation.To overcome the adverse effects of band-limitation, various alternatives for inverse filtering have been proposed. One class of alternatives is Lp-norm deconvolution, L1norm deconvolution being the best-known of this class.We show that for an exact convolutional forward model and statistically independent reflectivity and additive noise, the maximum likelihood estimate of the reflectivity can be obtained by Lp-norm deconvolution for a range of multivariate probability density functions of the reflectivity and the noise. The L∞-norm corresponds to a uniform distribution, the L2-norm to a Gaussian distribution, the L1-norm to an exponential distribution and the L0-norm to a variable that is sparsely distributed. For instance, if we assume sparse and spiky reflectivity and Gaussian noise with zero mean, the Lp-norm deconvolution problem is solved best by minimizing the L0-norm of the reflectivity and the L2-norm of the noise. However, the L0-norm is difficult to implement in an algorithm. From a practical point of view, the frequency-domain mixed-norm method that minimizes the L1norm of the reflectivity and the L2-norm of the noise is the best alternative.Lp-norm deconvolution can be stated in both time and frequency-domain. We show that both approaches are only equivalent for the case when the noise is minimized with the L2-norm.Finally, some Lp-norm deconvolution methods are compared on synthetic and field data. For the practical examples, the wide range of possible Lp-norm deconvolution methods is narrowed down to three methods with p= 1 and/or 2. Given the assumptions of sparsely distributed reflectivity and Gaussian noise, we conclude that the mixed L1norm (reflectivity) L2-norm (noise) performs best. However, the problems inherent to single-trace deconvolution techniques, for example the problem of generating spurious events, remain. For practical application, a greater problem is that only the main, well-separated events are properly resolved.

Notizen: We report on optoelectronic switching devices with high photoconductive gain which are sensitive to the polarization direction of the optical input. Due to the large polarization anisotropy of the absorption coefficient of ordered GaInP, an electrical output can be switched "on" and "off" by rotating the linear polarization of the input light. Depending on the external adjustment of the working point, the structure can operate as a polarization threshold switch or a polarization detector, respectively. First results exhibit a switching contrast of 25 dB and a contrast of the detector signal of 7 dB between two perpendicularly polarized incident light beams, respectively. © 1997 American Institute of Physics.

Notizen: We report on theoretical and experimental results on a novel metal-semiconductor-metal (MSM) photodetector with a backgate provided by a p-doped layer. The backgate allows extremely short sweep-out times for the holes, due to strong electric fields normal to the surface. Thus, long tails due to slow moving holes and screening of the external drift fields by hole space charge accumulation at high optical power, which lead to a degradation of the time response of conventional MSM photodetectors, are avoided. The high frequency performance measured up to 8 GHz in the time and frequency domain showed a significant reduction of the bandwidth limiting hole tail compared to standard MSM photodetectors. © 1994 American Institute of Physics.

Notizen: Transmission changes in a n-i-p-i doping superlattice have been measured. The structures were grown by a newly developed epitaxial shadow mask technique allowing to apply selective contacts to the n and p layers. For a voltage swing between only Upn=−4 and 0.8 V relative transmission changes of 65% below the band gap have been measured in a 2.66-μm-thick structure. The changes are larger than 50% in a wavelength range of more than 13 nm. The pure absorption changes are enhanced by a carrier- and field-induced-Bragg effect within the n-i-p-i crystal. The experimental results are in very good quantitative agreement with calculations.

Notizen: We report on the realization of an electro-optical switch on n-i-p-i doping superlattices for both bistable and step-like threshold operation at very low input power levels ((approximately-equal-to)200 pW) and with high opto-electrical gain (6.8×106). The configuration presented in this letter consists of a (two terminal) photodiode with negative current-voltage characteristic and a novel three-terminal photoconductive detector structure. The minimum optical switching power is determined by the reverse dark currents of the devices that are below pA. The switches are suitable for monolithical integration into arrays.

Notizen: We report on reverse biased photoconductive detectors with novel design and improved high-frequency performance. Taking advantage of the "giant ambipolar diffusion constant'' which has been observed previously in n-i-p-i doping superlattices very fast carrier transfer from the inner absorption area to the outer detection area is achieved. The combination of narrow contact spacings and small RC and diffusion time constants results in very high gain-bandwidth products ((approximately-greater-than)20 GHz) with adjustable 3 dB frequencies. © 1995 American Institute of Physics.

Notizen: We present a novel molecular beam epitaxial regrowth technique which provides a simple and convenient way for the in situ lateral structuring of the doping profiles and growth rates on a μm scale. We achieve excellent selective contacts to the respective doping layers for device dimensions varying from several 100 μm down to several μm. Keldysh based n-i-p-i modulator structures, fabricated with our new method, exhibit an on/off ratio of 6:1 for a voltage swing of 7 V without enhancement by additional Bragg mirrors.

Notizen: Wavelength-shifted GaAs/AlGaAs Fabry–Pérot ridge waveguide lasers were fabricated by vacancy-enhanced quantum well disordering using dielectric cap annealing. 500 μm long and 4 μm wide Fabry–Pérot lasers with emission wavelengths selectively shifted by 20 nm were integrated with unshifted lasers on the same chip, characterized and further compared with lasers fabricated from as-grown material. These investigations showed that the absorption edge of a single-quantum well double heterostructure can be selectively blueshifted after epitaxial growth without compromising diode laser performance. © 1995 American Institute of Physics.

Notizen: We report on optical logic gates and bistable opto-optical switches with high gain operating at very low threshold power. These devices are "smart pixels'' composed of n-i-p-i threshold switches and modulators. Very high values of 3×105 for the opto-optical gain ("fan-out'') and extremely low switching powers of less than 500 pW in combination with a broad hysteresis width of 200 pW are obtained. The switching power, the width of the hysteresis, and the opto-optical gain are all externally adjustable. The observed contrast ratio of 4.6 for the output signal corresponds to an optically induced voltage swing of 2.5 V.